With reference to FIG. 1, gas seals 100 are sometimes adapted for use on pumps, particularly to create a sealed relationship between the rotatable pump shaft 102 and the surrounding pump housing 104. The gas seal 100, which may be a single unit or, as shown in FIG. 1, a double unit, typically employs a pressurized barrier gas 106 which is supplied adjacent one periphery of the “lift off” region 114 that lies between the opposed seal faces 110, 112 (normally the outer periphery of the lift off region), while the opposite periphery of the lift off region (typically the inner diameter) is disposed in communication with the pumping (i.e. process) fluid 108. The barrier gas 106 is typically pressurized relative to the process fluid 108 and, in conjunction with pumping features such as grooves or the like provided on at least one of the opposed seal faces 110, 112, is effective for creating a gas film within the lift off region 114 between the opposed seal faces 110, 112 to maintain a small separation therebetween, while at the same time preventing the process fluid 108 from migrating between the opposed seal faces 110, 112.
In situations where the process fluid 108 is a liquid, and even in instances where the liquid has been termed cleaned, it has been observed that the liquid nevertheless can contain some quantity of small solid particles therein as contaminates. Furthermore, as the technology associated with gas seals and the life of such seals continues to improve, it has been observed that the small quantity of solids contained in “clean” liquids can create a problem with respect to the gas seal 100. In particular, it is believed that these solid particles tend to become trapped at a fairly high level of concentration in the liquid which gains entry into the lift off region 114 between the opposed seal faces 110, 112, typically from the inner periphery of the lift off region 114, and these solids tend to cause erosion or wear of one of the seal members adjacent said periphery of the lift off region 114.
This situation is made worse if for some reason the process fluid pressure should temporarily exceed the barrier gas pressure, due for example to a failure of the barrier gas pressurizing source, thereby causing the process fluid to tend to migrate into the lift off region 114.
One approach to excluding process fluids and contaminants from lift off mechanical seals is to provide a restrictive bushing and a positive-pressure clean fluid from an external source. However, this clean fluid and means of introduction into the seal can be costly and labor intensive to correctly install, and can consume considerable axial space in the seal chamber of a pump. In addition, many processes are averse to allowing dilution of pump process fluids with a clean external fluid.
Another approach is to provide a bushing with complex shapes that are configured to induce fluid flow patterns that direct contaminants in the process fluid away from the mechanical seal(s). Once again, such complex bushings can be expensive to manufacture and labor intensive to correctly install, and can consume considerable axial space in the seal chamber of a pump.
In addition, many mechanisms designed to exclude particulate contaminates from gas seals only work in cylindrical bore seal chambers, and will not function correctly if the bore or throat of the pump has been fouled by corrosion, erosion, or accumulated process solids.
What is needed, therefore, is a particulate exclusion mechanism for inhibiting particulate contaminates from reaching the lift off region of a lift off gas seal, where the particulate exclusion mechanism is inexpensive, easy to install and align, consumes very little axial space, and is not restricted to cylindrical bore seal chambers.